Magnetic shield material, production method thereof and color image tube assembling the material

ABSTRACT

PCT No. PCT/JP96/02673 Sec. 371 Date Mar. 10, 1998 Sec. 102(e) Date Mar. 10, 1998 PCT Filed Sep. 18, 1996 PCT Pub. No. WO97/11204 PCT Pub. Date Mar. 27, 1997Magnetic shield materials used in color picture tubes which have excellent inner magnetic shield characteristics and an excellent handling strength, method for producing the materials and color picture tubes produced by incorporating the materials are provided. The magnetic shield materials produced by subjecting hot rolled low carbon steel strips essentially consisting of equal to or less than 0.006 weight % of C, equal to or less than 0.002 weight % of N, equal to or less than 0.5 weight % of Mn, 0.5-2.5 weight % of Si, Fe as a balance and unavoidable impurities to a cold rolling and subsequently annealing the cold rolled steel strip at a temperature of 500-700 DEG  C. and applying a nickel plating thereto after annealing.

FIELD OF ART

The present invention relates to magnetic shield materials used in colorpicture tubes, a method of producing the materials and color picturetubes incorporating the materials, and more particularly, magneticshield materials used in color picture tubes, a method of producing thematerials and color picture tubes incorporating the materials which showan improved strength in handling.

BACKGROUND

A color picture tube such as a picture tube used in a color televisionset substantially comprises an electron gun and a fluorescent surfacewhich converts electron beams into an image. The inside of the picturetube is covered with magnetic shield materials for preventing electronbeams from being deflected by terrestrial magnetism.

As such magnetic shield materials, thin steel sheets on which a blacktreatment or a nickel plating is provided are used, wherein the thinsteel sheets are formed in a desired shape by bending and they aresealed to Braun tubes at a temperature of around 600° C. The steelsheets which are used as the magnetic shield materials are required tomeet favorable mechanical characteristics such as a favorableformability including bending and a handling strength capable ofpreventing the deformation of workpieces at the time of conveying themagnetic shield materials before or after the forming operation as wellas at the time of piling the workpieces, in addition to excellentmagnetic shield properties such as high permeability, low coercive forceand high shield efficiency.

To decrease the coercive force while increasing the permeability, thepresence of precipitation of carbon, nitrogen, carbide or nitride in thesteels which impedes the movement of a ferromagnetic domain wall must beminimized and the grain growth must be promoted while decreasing thegrain boundary. Although decreasing of carbon and nitrogen in the steelswhile increasing the grain growth of the steels provides an improvementof formability since the strength of the steels is decreased, at thetime of conveying the steel sheets or workpieces which are produced bybending, they tend to suffer from irregularities even when a slightimpact is applied to them or the workpieces tend to be deformed due tothe weight of the workpieces piled. Although the handling strength ofthe steel sheets can be enhanced by grain refining or addition of acertain amount of carbon and nitrogen into the steels so as toprecipitate carbide and nitride in the steels, such a method causesdeterioration of magnetic characteristics. In this manner, the steelsheets used as the magnetic shield materials must simultaneously meetthe excellent magnetic characteristics and the favorable handlingstrength which conflict with each other.

Conventionally, there have been proposed soft magnetic silicon steelsheets having an excellent handling strength as the materials which havemagnetic shielding characteristics. However, such steel sheets have notbeen in practical use, since it is difficult to apply a black treatmentrequired by color picture tubes to the soft magnetic silicon steelsheets. At present, as the magnetic shield materials, hyper-low carbonaluminium killed steel sheets provided with a black treatment and nickelplated steel sheets provided with a nickel plating have been practicallyused. Although these steel sheets have excellent magnetic shieldcharacteristics, they do not have a sufficient handling strength.

Accordingly, it is an object of the present invention to providemagnetic shield materials having excellent inner magnetic shieldcharacteristics and an excellent handling strength, and a method forproducing such materials and color picture tubes incorporating suchmagnetic shield materials.

DISCLOSURE OF INVENTION

The magnetic shield materials according to the present invention areproduced by subjecting hot rolled low carbon steel strips essentiallyconsisting of equal to or less than 0.006 weight % of C, equal to orless than 0.002 weight % of N, equal to or less than 0.5 weight % of Mn,0.5-2.5 weight % of Si, Fe as a balance and unavoidable impurities to acold rolling, annealing the cold rolled steel strip at a temperature of500-700° C. and providing a nickel plating thereto after annealing.

BEST MODE FOR EXECUTING INVENTION

According to the present invention, it is found that with an addition ofSi to hyper-low carbon steels, the tensile strength of the hyper-lowcarbon steels can be held equal to or more than 40 kg/mm² while thecoercive force thereof is held equal to or less than 1.2 oersted wherebythe magnetic shield materials used in color picture tubes havingexcellent magnetic characteristics and an excellent handling strength atthe same time can be obtained. The present invention is described indetail hereinafter in view of the following embodiment.

The hyper-low carbon steels used as the magnetic shield materials usedin color picture tubes according to the present invention are preferablyproduced by subjecting the steels to decarburization and denitrizationby a vacuum degassing so as to decrease carbide and nitride in thesteels and subsequently subjecting the steels to a hot rolling and acontinuous annealing to promote the grain growth in the steels.Furthermore, since carbide and nitride which are finely dispersed in thesteels prevent the movement of a ferromagnetic domain wall and thusdeteriorate the magnetic characteristics, elements which are to beincluded in the steels must be preliminarily restricted and the addtionamount thereof must be restricted as small as possible. Firstly, thereason for restricting kinds of elements included in the steels and theaddition amount of these elements is explained.

As for C, in case where an amount of C in the cold rolled steel sheetsis rich, carbide in the steels is increased so that the movement of theferromagnetic domain wall is hindered while the grain growth ishampered. Thus, it becomes difficult to lower the coercive force of thesteels. Accordingly, the upper limit of the amount of C should be 0.006weight %. The lower limit of the amount of C should be as low aspossible provided that the vacuum degassing can be effectively carriedout.

As for N, in case where aluminium killed steels are used as the magneticshield materials for the present invention, N reacts with solid-solutionstate aluminium in the steel to form fine AlN which deteriorates themagnetic characteristics. Accordingly, the amount of N should be equalto or less than 0.002 weight %.

As for Mn, the addition of Mn is necessary since Mn is bound to S in thesteel and fixes S in the steel as MnS to prevent the hot shortness.However, since corresponding to the decrease of the amount of Mn, themagnetic characteristics are increased, the amount of Mn should be equalto or less than 0.5 weight %.

As for Si, corresponding to the increase of amount of Si, the coerciveforce is lowered and the magnetic shield characteristics are improved.However, the elongation is decreased and the tensile strength isincreased thus lowering the formability. Although it depends on heattreatments which will be carried out after the cold rolling, with theamount of not less than 0.5 weight % of Si, the magnetic shieldcharacteristics and the handling strength required for the presentinvention can be obtained, while with the amount of more than 2.5 weight% of Si, the workability and formability are deteriorated. Accordingly,the upper limit of amount of Si should be 2.5 weight %.

The production processes of thin steel sheets which can be used as themagnetic shield materials are explained hereinafter.

Firstly, hyper-low carbon hot rolled strips having the above-mentionedchemical compositions which are produced by means of vacuum refining orvaccum degassing, are subjected to pickling so as to remove an oxidefilm produced during a hot rolling process. Subsequently, the hot rolledsteel strips are cold rolled at a rate of equal to or more than 70% soas to make the thickness of the steel strips 0.15-0.25 mm. With the coldrolling rate of less than 70%, when the steels strips are annealed afterthe cold rolling, the tensile strength of the strips becomes less than40 kg/mm² so that the handling strength required by the presentinvention cannot be obtained. Annealing should preferably be carried outat a temperature of 500-700° C. for 3 minutes-5 hours depending on therequired strength. When the annealing temperature is less than 500° C.,the steel strips are not sufficiently softened so that the workabilityof the steel strips becomes poor. Meanwhile, with a smaller amount ofSi, when the annealing temperature is high, the tensile strengh requiredby the present invention cannot be obtained. Furthermore, even with asufficient amount of Si, when the annealing temperature exceeds 700° C.,the tensile strength of equal to or more than 40 kg/mm² required for thepresent invention cannot be obtained even with a heating period of lessthan 3 minutes. Preferably, annealing should be carried out at atemperature of 550-650° C. for 5 minutes-2 hours corresponding to theamount of Si. Manner of annealing may either be a box annealing or acontinuous annealing depending on the heating temperature and theheating time.

After carrying out the above-mentioned annealing, the steel sheets aresubjected to an electrocleaning for the removal of grease and picklingin a diluted sulfuric acid so as to make the surface of the steel sheetsclean and activated. Subsequently, a nickel plating is applied to thesteel sheets making use of a nickel plating bath such as a Watt bath, anickel chloride bath, sulfuric acid bath, which are commonly used innickel plating technique. Increasing the plating amount is preferablefor satisfying corrosion-resistance but the smaller amount of platingshould be required in economical point of view. Terefore, the lowerlimit of the amount of nickel plating is 0.1 μm and the upper limitthereof is 5.0 μm.

EXAMPLE

The present invention is further explained in detail in view of thefollowing example.

Seven kinds of steels A, B, C, D, E, F and G respectively havingchemical compositions shown in Table 1 were prepared in the form ofslabs by a vacuum degassing and then were subjected to a hot rolling toproduce hot rolled steel sheets having a thickness of 1.8 mm. These hotrolled steel sheets were pickled in sulfuric acid and then weresubjected to cold rolling to produce cold rolled steel sheets having athickness of 0.15 mm. The cold rolled steel sheets were subjected to acontinuous annealing under 15 kinds of conditions shown in Tables 2-3 toproduce substrates for plating. These substrates for plating weresubjected to an alkali electrocleaning for the removal of grease andwere subjected to pickling in sulfuric acid. After pickling, a nickelplating having a thickness of approximately 1.3 μm was applied torespective substrates using a Watt bath having an ordinary bathcomposition. The coercive force of the annealed samples produced in theway mentioned was measured in such a manner that a first coil and asecond coil were wound around the samples and a magnetic field of 10oersted was applied to the samples. The tensile strength of the nickelplated steel sheets was measured by TENSILON.

Measured results are shown in Tables 2-3. It is understood that themagnetic shield materials according to the present invention has lowercoercive force, and at the same time higher tensile strength so that thematerials can preferably be used as the magnetic shield materials usedin color picture tubes. On the contrary, Comparative Example A-1 and A-2failed to obtain sufficient magnetic shield characteristics and asufficient tensile strength, while Comparative Example G-1 and G-2showed excessively high tensile strength compared to the tensilestrength required for the present invention so that they had poorformability.

Because of excellent magnetic characteristics and a handling strength,the magnetic shield materials of the present invention can be used notonly as inner shield materials of color picture tubes but also as framematerials which are interposed between the inner shield materials andshadow mask materials so as to fixedly secure them to panels. Samplecodes (Alphabet-Number) in Tables 2 and 3 indicate magnetic shieldmaterials produced by using kinds of materials (left portion of thematerial codes ) shown in Table 1 and varying conditions (right portionof the material codes).

                  TABLE 1                                                         ______________________________________                                        (wt %)                                                                          kind of                                                                       steel C N Mn Si Fe                                                          ______________________________________                                        A        0.006  0.002      0.47 0.10    balance                                 B 0.005 0.002 0.39 0.50 balance                                               C 0.006 0.002 0.42 1.01 balance                                               D 0.006 0.002 0.47 1.52 balance                                               E 0.005 0.002 0.44 1.99 batance                                               F 0.006 0.002 0.41 2.49 balance                                               G 0.006 0.002 0.42 3.01 balance                                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        annealing                                                                       condition                                                                         tempera-        coercive                                                                            tensile                                             Sample ture time force strength                                               code (° C.) (min) (Oe) (kg/mm2) Classification                       ______________________________________                                        A-1   500      300    1.22  18     Example                                                                              present                               A-2 700 3 1.23 27 Example invention                                           B-1 500 300 1.19 40 Example                                                   B-2 700 3 1.20 40 Example                                                     C-1 500 300 1.15 42 Example                                                   C-2 700 3 1.15 41 Example                                                     D-1 450 300 1.11 57 Example                                                   D-2 500 300 1.11 50 Example                                                   D-3 550 210 1.10 48 Example                                                   D-4 700 3 1.10 45 Example                                                   D-5   750      3      0.09  33     Comparative                                       Example                                                                ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        annealing condition                                                                          coercive                                                                              tensile                                                Sample                                                                              temperature                                                                             time   force strength                                           code (° C.) (min) (Oe) (kg/mm2) Classification                       ______________________________________                                        E-1   500       300    1.07  60     Example                                                                              invent-                              E-2 700 3 1.07 52 Example ion                                                 F-1 500 300 1.02 67 Example                                                   F-2 700 3 1.02 60 Example                                                   G-1   500       300    0.98  75     Comparative                                 G-2 700 3 0.98 70 Example                                                   ______________________________________                                    

INDUSTRIAL APPLICABILITY

The magnetic shield materials according to the present invention aremagnetic shield materials used in color picture tubes which are producedby subjecting a hot rolled low carbon steel strip essentially consistingof equal to or less than 0.006 weight % of C, equal to or less than0.002 weight % of N, equal to or less than 0.5 weight % of Mn, 0.5-2.5weight % of Si, Fe as a balance and unavoidable impurities to a coldrolling and annealing the cold rolled steel strip at a temperature of500-700° C. and applying a nickel plating thereto after annealing. Thematerials having a low coercive force have excellent magnetic shieldcharacteristics and a high handling strength so that the materials arepreferably be used as the magnetic shield materials used in colorpicture tubes.

We claim:
 1. A magnetic shield material used in color picture tubesproduced by subjecting hot rolled low carbon steel strip essentiallyconsisting of equal to or less than 0.006 weight % of C, equal to orless than 0.002 weight % of N, equal to or less than 0.5 weight % of Mn,0.5-2.5 weight % of Si, Fe as a balance and unavoidable impurities to acold rolling, annealing the cold rolled steel strip at a temperature of500-700° C. and applying a nickel plating thereto after annealing. 2.Method of producing magnetic shield material used in color picture tubescomprising subjecting a hot rolled low carbon steel strip essentiallyconsisting of equal to or less than 0.006 weight % of C, equal to orless than 0.002 weight % of N, equal to or less than 0.5 weight % of Mn,0.5-2.5 weight % of Si, Fe as a balance and unavoidable impurities to acold rolling, annealing the cold rolled steel strip at a temperature of500-700° C. and applying a nickel plating thereto after annealing.
 3. Acolor picture tube incorporating a magnetic shield material produced bysubjecting a hot rolled low carbon steel strip essentially consisting ofequal to or less than 0.006 weight % of C, equal to or less than 0.002weight % of N, equal to or less than 0.5 weight % of Mn, 0.5-2.5 weight% of Si, Fe as a balance and unavoidable impurities to a cold rolling,subsequently annealing the cold rolled steel strip at a temperature of500-700° C. and applying a nickel plating thereto afer annealing.
 4. Amethod according to claim 2 wherein said hot rolled low carbon steelstrip is obtained by decarburization and denitrization of steel byvacuum degassing, followed by hot rolling.
 5. A method according toclaim 2 wherein said cold rolling is carried out at a rate equal to ormore than 70% to provide the cold rolled steel strip with a thickness of0.15-0.25 mm.
 6. A method according to claim 5 wherein said annealing iscarried out at a temperature of 550-650° C. for about 5 minutes to about2 hours.
 7. A method according to claim 6 wherein said nickel plating isapplied in a thickness of 0.1 μm to 5.0 μm.
 8. A magnetic shieldmaterial for a color picture tube comprising a low carbon steel stripconsisting essentially of up to 0.006 weight % of C, up to 0.002 weight% of N, up to 0.5 weight % of Mn, 0.5-2.5 weight % of Si, and theremainder Fe and unavoidable impurities, said steel strip having atensile strength of at least 40 kg/mm² and a coercive force no greaterthan 1.2 Oersted, said steel strip having a nickel plating layer thereonhaving a thickness of 0.1 μm-5.0 μm.
 9. A color picture tubeincorporating the steel strip of claim 8 as a shield material.